Piston type automotive air conditioning compressors have a generally cylindrical cylinder block with a plurality of cylinder bores arrayed around, and parallel to, a central axis of the block. A piston in each cylinder bore is reciprocated back and forth by one of two main types of drive mechanisms, a wobble plate or a swash plate. Each drive mechanism is a plate that is driven about the axis of the cylinder block at a tilt angle or fixed angle of nutation so that the edge of the plate reciprocates axially back and forth relative to the pistons. When connected to the pistons, the pistons are correspondingly driven back and forth in their bores. Obviously, the piston to plate connection will have to allow relative slipping, since the pistons cannot rotate with the plate. In the case of a wobble plate, part of the plate itself is allowed to slip relative to another part of the plate, which is sometimes referred to as a slipper foot design. In the case of the swash plate, the plate is solid, and the edge of the plate slips through a pair of semi spherical bearings that ride in a socket at the back of the piston. The shape and manufacture of the piston is greatly affected by whether the drive mechanism is the wobble or swash plate type. In general, piston manufacture and design is significantly more difficult in the case of a swash plate, for reasons described below.
Before turning to the state of the current art in piston shape and manufacture, it is useful to turn to FIG. 8 of the drawings to get a general understanding of the framework within which a piston designer would work. As the piston moves in the bore, it's outer surface slides and rubs over the inner surface of the bore, and the two interfit closely. At or near top dead center, the piston is almost entirely inside the bore, and piston guidance, that is, the degree to which the piston axis is kept on the bore axis, is good. As the piston retracts, much of its outer surface is pulled out of the bore. At that point, other mechanisms have to be relied upon for piston guidance. Nevertheless, the piston designer is compelled to design a piston that has as much piston outer surface area in contact with as much of the bore inner surface as possible, or, at least, as much as is possible within the constraints of piston manufacturability and weight. Now, FIG. 8 schematically represents what may be thought of as a potential outer surface envelope for a theoretical piston, a piston which would be located at the lowermost or "6 o'clock) position in a compressor cylinder block that was cross section in a 12 o'clock-6 o'clock plane. The outer surface envelope represents the total surface area that can possibly be in contact with the bore, and breaks it down into six different portions. The front and back portions, F and B, are simple cylinders, which are significantly shorter than the total bore length, but with continuous outer surfaces that contact a total 360 degrees worth of the bore inner surface. The back portion B is not particularly significant to piston guidance in the cylinder bore per se, although it has implications for piston strength. The back portion B is simply not in the cylinder bore for very long in any given stroke, while the front portion F is always inside the bore. The rest of the potential envelope, which is the majority of it, is divided up into a semi cylindrical outer portion O, which would face radially outwardly of the cylinder block, an opposed semi cylindrical inner portion I, and two opposed semi cylindrical side portions S. Each of these portions may be conceived as subtending about 90 degrees. These are shown exploded out for purposes of illustration. In addition, a center axis A is indicated, as well as a central plane P that would run through A and bisect the inner and outer portions O and I. A double headed arrow indicates a direction perpendicular to A, moving through or toward the side portions. While this may seem over analytical, it provides a unique and novel framework for surveying and cataloging the myriad piston design approaches that have been taken to date, although the designers were not likely thinking consciously in terms of such a theoretical design framework at the time.
The simplest piston design of all would be no more that a solid cylindrical plug or head that corresponded to the front portion F. In fact, many old and current piston designs, in wobble plate compressors, are exactly that. This is possible because, in a wobble plate, the short piston head is connected to the slipper foot portion of the wobble plate by a thin rod with a spherical joint at each end. This simple piston shape can be easily turned on a lathe. A variation of this simple design may be seen in U.S. Pat. No. 4,526,516 to Swain et al. issued Jul. 2, 1985, where the piston has a short, solid head at the front, and a longer cylindrical skirt extending axially back from the head. A relatively thin center post is fixed to the slipper foot of the wobble plate with a spherical headed post. This piston design, too, can be lathe turned. It is substantially hollow, and therefore light, but has essentially the entire potential surface envelope presented to the bore. However, this type of piston design is not practical in a swash plate piston, as will be seen. Another possible approach is to put a forwardly extending sleeve or skirt extending forwardly of the piston head, rather than extending back, a design that could also be lathe turned. This, however, would require a greater total cylinder block length.
A swash plate piston presents unique manufacturing challenges that affect how much of, and how easily, the entire potential surface envelope of the piston can be used. A typical swash plate piston may be seen in co assigned U.S. Pat. No. 5,461,967 to Burkett et at. issued Oct. 31, 1995. As shown there, the piston 20 is integral and solid, but in terms of the surface envelope as defined above, it utilizes only the front portion F (that being the outer surface of the front end 34) and the outer portion O (called out as an outer surface 36). This piston 20 is more than just a front plug or head, but really adds only the outer surface 36 for extra cylinder bore contact. While much of the potential piston outer surface contact envelope is thus not utilized (most notably the inner portions I as defined above), it is not so important in the design disclosed, which has a unique piston control ring 42 to help guide the piston 20 and to make up for the absence of an inner portion I. Furthermore, the piston 20 at least has the advantage of being easily and relatively inexpensively manufactured, as well as being relatively light and low mass. While the patent does not speak a great deal to how the piston 20 would be manufactured, those skilled in the art will note that the shape of piston 20 is such that none of it's outer surfaces present a concavity, as seen in the direction of the arrow in FIG. 8, except for the ball socket, a non avoidable concavity which must be machined out in any piston of the same general type. Therefore, the rest of the piston body could be forged or east (at least to a near net shape) with only two dies or molds, which could move together or apart in the direction of the arrow in FIG. 8. Only final finish surface of the bore contact surfaces 34 and 36 (and of the ball socket) would be needed. At the far end of the spectrum, the piston design shown in U.S. Pat. No. 5,174,728 to Kimura et at. issued Dec. 29, 1992 utilizes the entire outer envelope, having a cylindrical body 12 with a complete, outer cylindrical surface that is closed at front and back, but which is entirely hollow. This is the most difficult and expensive design of all to manufacture, however, and must inevitably be formed of at least two pieces welded together, as a closed canister would be. The interior must also be vented to prevent pressure differentials from crushing the thin walled and hollow outer body.
In between the two piston design extremes of head only and two piece, hollow canister are other designs which attempt to keep a one piece integral structure, while retaining as much outer surface area as possible, but eliminating as much solid material volume as possible for weight reduction. These are competing purposes, obviously, and proposed designs fall short either by failing to provide critical piston outer surface portions, or by being very difficult to manufacture, or both. One such design is shown in U.S. Pat. No. 5,382,139 to Kawaguchi et al. issued Jan. 17, 1995, in which piston 9 is concave, as opposed to truly hollow, and is missing the entire outer surface portion O, being open at that area instead. The design also has an internal concavity in the head portion that would prevent it from being die east with only two mold halves, and which would require instead that the piston interior be either lost core east or internally machined out. In Japanese Laid Open patent application 7-189900, several variations of the same basic shown in the '139 patent. In FIG. 6 of the Japanese application, the piston body is concave, on either one or both sides, so as to eliminate weight, but this also eliminates any outer surface area on at least one side portion S. In most of the embodiments disclosed, outer surface area is absent on both of the side portions S defined in FIG. 8. One embodiment is completely asymmetrical, having surface area all on one side portion S only, and none on the other, giving a C shaped cross section. (See FIG. 6 of 7-189900) In addition to not having symmetrical support on both side portions S, the piston is, at best, concave, not truly hollow. That is, as viewed along the arrow of current FIG. 8, solid material would be seen, either on one side, as in FIG. 6, or in the middle, at a central web centered on the plane P. This is clearly not as light or mass efficient as a completely hollow design would be, that is, a design in which no solid piston body material was seen or encountered when moving along the arrow shown in FIG. 8.